Pacific Intermediate Water

Pacific Intermediate Water
Name	Pacific Intermediate Water
Region	Pacific Ocean
Depth	300–1500 m
Temperature	4–12 °C
Salinity	34.2–34.8 PSU
Density	26.8–27.6 σθ
Formation	Subantarctic Mode Water, North Pacific Intermediate contributions

Pacific Intermediate Water is an intermediate water mass occupying mid-depths of the Pacific Ocean that influences thermohaline structure between surface currents like the North Pacific Gyre and deep flows such as the Pacific Deep Water. It interacts with large-scale features including the Kuroshio Current, the California Current, and the Equatorial Pacific, and plays a key role in modulation of climate variability associated with El Niño–Southern Oscillation and coupling to the Atlantic Meridional Overturning Circulation via inter-basin exchange. Research on this water mass connects institutions such as the Scripps Institution of Oceanography, the Woods Hole Oceanographic Institution, and programs like the World Ocean Circulation Experiment.

Definition and Characteristics

Pacific Intermediate Water is defined as a mid-depth water mass within the Pacific Ocean characterized by temperature, salinity, and potential density ranges intermediate between surface and deep waters, typically found at 300–1500 m beneath the North Pacific Subtropical Convergence and across the Southern Ocean inflow pathways. Its core properties often intersect with criteria used for identifying Subantarctic Mode Water, Central Water, and North Pacific Intermediate Water in hydrographic sections produced by expeditions such as the International Geophysical Year campaigns and modern cruises from the GEOTRACES program. Oceanographers from the National Oceanic and Atmospheric Administration employ water mass analysis methods derived from classical studies by researchers associated with the Lamont–Doherty Earth Observatory.

Formation and Sources

Formation of Pacific Intermediate Water involves contributions from southward incursions of Subantarctic Mode Water and northward input from Antarctic Intermediate Water transported via the Drake Passage and the Antarctic Circumpolar Current. Local processes near marginal seas like the Sea of Okhotsk and the Bering Sea add brine-modified intermediate waters influenced by winter convection documented in studies by the Japan Agency for Marine-Earth Science and Technology and teams collaborating with the National Institute of Water and Atmospheric Research. Exchange across the Indonesian Throughflow and mixing near the Philippine Sea and Kuril Islands further modulate source signatures recognized in tracer work undertaken by researchers at the Max Planck Institute for Meteorology.

Distribution and Circulation

Pacific Intermediate Water forms a broad, continuous layer extending from mid-latitudes into the subtropical and equatorial Pacific, following pathways influenced by the North Equatorial Current, the Oyashio Current, and the East Australian Current. Its circulation links to subduction zones beneath the Subtropical Gyres and recirculation within basins described in model studies by the National Center for Atmospheric Research and the European Centre for Medium-Range Weather Forecasts. Interactions with mesoscale features such as Eddy Kuroshio rings and boundary currents near the Gulf of Alaska produce regional variability noted in observations from platforms operated by the Alaska Fisheries Science Center.

Physical and Chemical Properties

Physically, Pacific Intermediate Water exhibits temperatures generally from about 4 to 12 °C and salinities near 34.2–34.8 PSU, with potential density anomalies around 26.8–27.6 σθ, values that oceanographers map using conductivity–temperature–depth casts from research vessels like the RV Knorr and RV Investigator. Chemically, it carries tracer signatures including oxygen minima and nutrient maxima shaped by remineralization gradients and biological export measured in programs such as Global Ocean Ship-based Hydrographic Investigations Program and Scripps's HOT cruises; isotopic and transient tracers used by teams at the Lamont–Doherty Earth Observatory and the NOAA Pacific Marine Environmental Laboratory help distinguish contributions from Antarctic Intermediate Water and locally modified sources. Biogeochemical contrasts in dissolved inorganic carbon and alkalinity across the intermediate layer are important for studies conducted under initiatives like the International Ocean Carbon Coordination Project.

Ecological and Climatic Significance

Pacific Intermediate Water influences nutrient supply to the euphotic zone through upwelling and mesoscale exchange affecting ecosystems associated with the California Current System, the Peruvian Upwelling, and pelagic habitats supporting fisheries managed by agencies such as the Pacific Fishery Management Council. Its variability modulates climate teleconnections associated with the El Niño–Southern Oscillation, influences heat uptake relevant to studies by the Intergovernmental Panel on Climate Change, and participates in carbon sequestration processes assessed by the Global Carbon Project. Changes in intermediate water properties have implications for species ranges documented by researchers at the Monterey Bay Aquarium Research Institute and for regional climate impacts evaluated by centers like the Potsdam Institute for Climate Impact Research.

Observation and Measurement Methods

Observation of Pacific Intermediate Water uses hydrographic surveys with CTD sensors aboard research vessels including the RV Sonne and RV Ka'imikai-o-Kanaloa, autonomous profiling floats from the Argo program, and moored arrays deployed by consortia such as the Pacific Islands Fisheries Science Center. Tracer analyses employ techniques from laboratories at the Scripps Institution of Oceanography and the Woods Hole Oceanographic Institution for nutrients, oxygen isotopes, chlorofluorocarbons, and radiocarbon used in conjunction with numerical models developed at the National Oceanography Centre and the Geophysical Fluid Dynamics Laboratory to infer water mass pathways and residence times. Satellite altimetry from missions like TOPEX/Poseidon and Jason-3 complements in situ observations by resolving mesoscale dynamics that redistribute intermediate water properties.

Category:Oceanography